1. Field of the Invention
The present invention relates to arbitration of an access to an optical communication bus and use thereof. More particularly, the present invention relates to arbitration of access for one of a plurality of optically interconnected nodes that share the communication bus.
2. Related Art
FIG. 1(a) is a diagram showing an exemplary configuration of an optical communication bus for which a number of blade servers are optically interconnected (number of nodes).
Commonly owned, unpublished Japanese Patent Application No. 2010-149373 (Attorney Docket No. JP920100021JP1) describes an optical communication bus for a backplane of blade servers. Such an optical communication bus is depicted in FIG. 1(a) above.
In the above application, the communication distance between any pair of blade servers to be inserted arbitrarily into a backplane, for example, up to six blade servers, is set equal.
As schematically shown in FIG. 1(b), the portion of the communication bus is shared among the optically interconnected blade servers (nodes). Specifically, accesses to (or use of) the communication bus are shared in a portion indicated by M. This configuration is called a star topology because of its radial branching as seen in L0 to Ln or N0 to Nn.
As schematically shown in FIG. 1(b), three communication paths, i.e., a communication path L0-M-N0, a communication path L1-M-N1, and a communication path L2-M-N2 are all set to have an equal distance. Schematically modified from the view FIG. 1(c) to FIG. 1(d), the communication paths are formed as a fiber sheet or an optical waveguide in the backplane.
Between blade servers (nodes) with an equal communication distance that are set in the above manner, a Wave Division Multiplex (WDM) technique using a number of wavelengths is used for selecting a data source and a data destination. However, it is known that WDM devices are expensive.
An alternative to WDM is a Time Division Multiplex (TDM) technique, which transmits data in a time division manner while using the same wavelength.
FIG. 2 is a diagram showing a Passive Optical Network (PON) as a conventional technique using a TDM optical link.
In a PON, a timing at which each node can transmit data is preassigned to the node as a timeslot. For a number of subscriber optical network units (ONUs) to use a central optical line terminal (OLT), time division is used with a guard time secured.
However, for a blade server, an approach of contending for the use of a bus by request from each blade is desirable from the viewpoint of Bandwidth on Demand.
In an optical backplane, clock timing is embedded in data. Therefore, a receiver side performs Clock Data Recovery (CDR). For effective use of the bus, a CDR device on the receiver side needs to enter a lock state and retrieve the clock as soon as possible after the start of transmission.
While the lock time depends on various parameters such as the CDR type, how the input data changes, and how the frequency shifts, the lock time is typically about several tens to several hundreds of nano seconds.
Generally, an optical link for one-to-one or one-to-many connection always maintains the lock state by sending an idle signal even when no data is being transmitted. However, this technique cannot be used for an optical bus because there are several light-emitting sources and they should not arbitrarily transmit idle data.
FIG. 3(a) is a schematic diagram showing a waveform with a DC balance kept.
FIG. 3(b) is schematic diagram showing a waveform with a DC balance lost.